An instrument for testing circuit boards is known from U.S. Pat. No. 5,254,953, wherein a first electrode and a component being tested act as plates of a capacitor of which the capacitance is determined in the instrument by the change in capacitance. For that purpose the measuring amplifier must measure the current in a conductor between the first and second electrodes, latter in the known instrument of the species acting as the ground of the circuit on a circuit board subjected to testing.
This instrument allows component-testing for instance of circuit boards. Illustratively, if the component being tested does not make proper contact due to inadequate soldering or due to an electrical discontinuity, then only the substantially lower capacitance subtended with more remote components, for instance, the line on the other side of the discontinuity, shall be determined.
The instruments of this species offer the advantage of not requiring an electrical, hence mechanical contact between the first electrode and the component. Contact surfaces of high conductivity are not required at the component.
Illustratively, oxide layers do not interfere. Moreover, the measurement may be carried out passing through insulating layers or non-conducting housings.
Because the measurement gap is included in the measured capacitance, and provided the first electrode is accurately positioned, it is possible to ascertain geometric deviations of the component being tested, such as would be caused by bending, oblique assembly or the like. Furthermore, socket contacts can be measured that are designed only for single use and, therefore, may not be mechanically stressed, for instance by a tester plug.
The capacitive testing procedure of the known instrument incurs the drawback of requiring a current in the circuit for changing the change on the capacitor. In complex circuits, such a current also usually passes through other components of which any deviations will be included in the test result.
Because the measuring amplifier determines the current going to circuit ground, there always is additional capacitance between the first electrode and the circuit ground in parallel with the capacitance to be measured. Moreover, this spurious parallel capacitance is usually larger than that which is to be measured, and deviations in this parallel capacitance with ground entail substantial deviations. Also high test sensitivity is required in order to allow determining the small deviations of the measurement capacitance within the large total capacitance (measurement capacitance+much larger capacitance to ground).
A test instrument of this kind is known from the German patent document A1 26 39 831. In this known instrument, the field generated by the component when electrically loaded by a drive unit is measured with a field probe. In this known design, the field probe is a coil that responds solely to the field constituents of the component-generated electromagnetic field. However, magnetic fields are generated only if there is a current in the component. Accordingly, the drive unit must be designed so as to generate a current in the component. However, this feature entails drawbacks similar to those described above in relation to the capacitive method. All adjacent circuit components also passing a current might interfere. Components that somehow were improperly connected and, hence, do not pass a current cannot be tested at all. Furthermore, the inherent coil size limits the spatial resolution of this testing procedure.
An objective of the invention is to create a kindred instrument retaining the advantages of contact-less measurement and offering greater sensitivity in detecting deviations of the component being tested while being less sensitive to other circuit deviations.
In the instrument according to the present invention, the two electrodes are used to determine a voltage between two sites of the electrical field generated by the component under applied voltage. A magnetic field, and hence a current in the component, is not required. Accordingly, it is enough to make contact by one conductor alone with the component or a circuit fitted with a component. DC voltage may be used, and hence a purely electrostatic field is then generated. The two electrodes constitute a field probe allowing to survey with high accuracy the field geometry around the component. Both electrodes, preferably however one electrode near the component, may be displaced. Near the component to be tested, the field essentially depends only on that component. If the component illustratively is disconnected from the drive voltage because of inadequate soldering or a break in a conductor, there will be marked field deviations. These deviations can be easily ascertained relative to the nominal state that, for instance, was ascertained by a previous measurement on a properly operating circuit board. Geometric deviations of the component being tested, for instance skewness, bending etc. are very easily detected by the resultant field changes. Because of their geometrically remote positions, other components affect the field very little, especially as regards the near field around the component to be measured. The measurement procedure does not necessarily entail currents in the circuit and their ensuing spurious measurement errors. As in the kindred state of the art, any contact between the electrodes and the circuit is eliminated. Thus, surfaces offering good contacts are not needed. Measurement can be carried out across insulated surfaces or through plastic housings, for instance inside an IC imbedded in a plastic case. Because field asymmetries are detected especially well, the heretofore unsolved problem of spotting electrolytic capacitors mounted at the wrong polarity is now solved in a simple manner. The drive unit may apply special voltages to the component, for instance DC, which allows circumventing capacitor problems in the circuit. However, pulses, for instance AC voltages of arbitrary frequencies, also may be used. Even the electric field may be determined which is generated by the components at rated circuit operation. In this manner the instrument of the invention also may be used as a function tester. When the field probe of the invention offers appropriate geometric resolution, it will allow observing even the switching processes within an IC.
The second electrode may be mounted anywhere in the electric field, for instance far from the site of measurement. In this case the first electrode only need be moved from test site to test site. If both electrodes are near the component to be tested, then the electric near field, where the field intensity differentials are very high, shall be measured very accurately, free from the fields of other components.
The field probe can be mounted in very simple manner to the end of a shielded cable. As a result, field measurements can be carried out with a spatial resolution of less than 1 mm.
As a result of further features of the invention, when carrying out near-field measurements on components, it is possible to avoid electric contact with these components, even in the case of .position deviations that would entail interfering electrical contact with the circuit.
The first electrode also may simultaneously sense several components. Provided there is appropriate excitation, i.e. drive, for instance in sequence, of the individual components, each time the other components being at ground, for instance by mans of the drive unit, a single position of the first electrode will allow measuring a. substantial number of components. This feature is especially advantageous when the components are substantially identical geometrically and electrically, for instance if they were the contact pins of a plug or the connection pins of an IC.
The drive unit may apply a DC voltage and generate an electrostatic field that can be accurately measured by the instrument of the invention. Advantageously, however, a pulsed voltage can be applied, or an AC voltage, which is more easily detected by the measuring amplifier.
Advantageously, low-frequency pulse trains are used which are outside the conventional frequency range being radiated by nearby test equipment and computers.
Preferably, the pulse shape is triangular. This feature offers the advantage that differentiation by the test instrument""s capacitors will shape them into rectangles which are more easily processed by this instrument.
The measuring amplifier preferably suppresses DC voltages. Electrostatic fields, such as are generated by electrostatic charges accumulating in the vicinity of the test site and that may lead to spurious test results, are thereby suppressed. Advantageously, the test instrument is designed such that it suppresses any interference frequencies, such as line power (50-60 Hz range), and frequencies that are generated by nearby line-powered equipment, and other interfering frequencies near the measurement site, for instance by computer CPUs in the 100 MHz range, from robot control motors and the like.
In further accordance with the present invention, the electrical field near one or several components on a circuit board is mapped quantitatively by moving the electrode in its geometric configuration.
Alternatively, a large number of electrodes in a matrix array acting as the first electrodes are mounted, for instance parallel by their plane surfaces, to the circuit board to be tested. The matrix electrodes are connected, by means of an appropriate system, to the measuring amplifier and, in this manner, provide a static, immobile construction affording spatial resolution of the field generated by the component.
By means of column-and-row electrodes, of which every pair is crossing and connected to the measuring amplifier, allowing multiplex control, surface resolution of the field is possible. This is because, when connecting two electrodes of the matrix array, only these particular two are detecting at this time. The connected electrodes have their highest sensitivity at the location where they cross, whereby this crossing point indicates its local field intensity. By multiplex-controlling sequentially all column-and-row electrodes, the field intensities at all crossing points can be determined.